1. Field of the Invention
This invention relates to a laser printer. More particularly, it relates to an improved laser printer in which the degradation of the image quality attributable to a property of a photoreceptive means can be compensated.
2. Description of the Prior Art
A laser printers comprises a laser beam emitting device, and a photoreceptive means on which latent images are formed by irradiating a laser beam from the laser beam emitting device. The image quality of such a laser printer largely depends on the conditions of the photoreceptive means. The major causes of changing the conditions of the photoreceptive means include the deterioration of the photoreceptive means and the changes in temperature of the photoreceptive means. If the photoreceptive means deteriorates or its temperature changes from the normal conditions, the potential of a portion of the photoreceptive means irradiated by a laser beam (hereinafter, such a portion is referred as "a laser-irradiated portion") varies even when the optical output level of the laser beam is fixed at a constant value. The variation of the potential of a laser-irradiated portion produces a change in density of a printed image in the negative development system, or a fog in the positive development system.
Countermeasures in the prior art against the deterioration of image quality involve improvements of the photoreceptive means itself and of ambient conditions, such as an extension of the service life of the photoreceptive means by improving its composition or structure, betterment of temperature characteristics of the photoreceptive means, and amelioration of ambient conditions (e.g., a cooling means is provided for maintaining the temperature of the photoreceptive means in a predetermined range).
As laser printers have become more compact yearly and their parts are densely assembled, it is difficult to overcome the deterioration of image quality by improving the photoreceptive means itself or ambient conditions.